# Crystal

### From Online Dictionary of Crystallography

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- | Here <math>\textbf{a}_{i}</math> and <math>h_{i}</math> are the reciprocal lattice vectors and integer coefficients respectively. | + | Here <math>\textbf{a}_{i}</math> and <math>h_{i}</math> are the reciprocal lattice vectors and integer coefficients respectively and the number ''n'' is the minimum for which the positions of the peaks can be described with integer coefficient <math>h_{i}</math>. |

The conventional crystals are a special class, though very large, for which ''n'' = 3. | The conventional crystals are a special class, though very large, for which ''n'' = 3. |

## Revision as of 15:49, 3 April 2009

## Definition

A material is a crystal if it has **essentially** a sharp diffraction pattern. The word **essentially** means that most of the intensity of the diffraction is concentrated in relatively sharp **Bragg peaks**, besides the always present diffuse scattering. In all cases, the positions of the diffraction peaks can be expressed by

Here and *h*_{i} are the reciprocal lattice vectors and integer coefficients respectively and the number *n* is the minimum for which the positions of the peaks can be described with integer coefficient *h*_{i}.

The conventional crystals are a special class, though very large, for which *n* = 3.

## See also

*Acta Cryst.* (1992), **A48**, 928. The definition of a crystal appears in the **Terms of reference** of the IUCr commission on aperiodic crystals